The art of manufacturing laminated components, such as rotor cores, is well known in the electric motor industry. Generally, laminas are blanked from successive regions of a sheet metal strip by a progressive die assembly which includes several stations at which numerous operations are performed on the laminas. For example, the die assembly includes stations for punching numerous holes through the laminas, blanking the laminas, stacking the laminas, and interlocking the laminas into a solid core. Unfortunately, the sheet metal often includes a non-uniform thickness such that, when the laminas are stacked together in the same orientation, the thickness discrepancies accumulate to create a "parallelism" error. In other words, the thick portion of each lamina will be placed in the same position in the stack such that one side of the stack will be larger than the other, causing the stack to tilt or lean in the direction of the thin portion of each lamina.
One solution to this problem is to provide relative rotations between laminas in the stack to thereby distribute any thickness variations evenly about the stack. For example, U.S. Pat. No. 4,445,272 to Bruhn et al. discloses a method and apparatus for constructing a laminated rotor in which laminas are rotated a large increment, such as 90.degree., to evenly distribute any thickness variations in the sheet stock throughout the rotor core. In addition, U.S. Pat. No. 4,619,028 to Neuenschwander discloses a method and apparatus for manufacturing laminated rotor and stator cores in which the thickness of the sheet stock is measured to determine the appropriate degree of rotational correction necessary to compensate for any thickness deviations. Based upon the measured thickness deviation and other information, a rotational correction is imposed upon at least one of the laminas within the stack to compensate for the thickness variations. If desired, a small skew angle of rotation can be applied to each lamina in the stack in addition to any rotational corrections. The rotational corrections can also be applied to a rotor core having a counterbore and can be made to any lamina in the counterbore or at the juncture between the counterbore and shaft-hole segments of the rotor core.
A significant problem with rotating the laminas by a large increment is that the central shaft hole in each lamina is often not precisely concentric or in the center of each lamina. As such, when adjacent laminas are relatively rotated by a large angle, the shaft holes will be misaligned and will overlap due to the lack of concentricity. The effective size of the shaft hole will be reduced by a variable amount depending on the severity of the concentricity problem. The close tolerances necessary within the shaft hole are thus impossible to maintain, making rotation of the laminas within the stack impractical and undesirable.